Pay TV systems generally fall into one of two categories, in which either subscribers pay a monthly fee to view all signals transmitted on certain channels during a certain time period, or subscribers "pay per view" in which selected programs may be viewed by certain subscribers who pay a fee for each requested program. In the first category, filters may be removed from the transmission lines of authorized subscribers, or a descrambling signal may be downloaded from a head end to decoders in subscribers' terminals. In the second category, decoding signals restricted to particular programs are transmitted either via a telephone line or via the cable network to addressable decoders.
It is very expensive to send a serviceman to remove or to add filters locally at each subscriber location in a large population each time subscribers sign on or cancel service, or neglect to pay for the service. Further, the requirement to address each subscriber terminal by individual address in order to provide a descrambling code for specific programs or for specific time periods has been found to be a less than desirable system, since it utilizes scarce bandwidth and it is slow since it requires downloading of codes over a long period of time.
Further, such systems have not been capable of being used with common cable TV networks used by multiple cable TV companies, since a subscriber to one cable TV company could telephone another and obtain a download of a descrambling code thus receiving signals to which the subscriber is not entitled, thus depriving the first cable company of revenue from that subscriber.
Moreover, because of the relatively poor security, such systems are not feasible for use to provide other than cable TV services, for example banking services, personal service or merchandise ordering services, etc., in which information as to accounts, account statements, etc. would be desired to be displayed on the television display of the subscriber.